Heat energy is a form of energy that can be utilised for a wide variety of purposes. Many processes depend for their operation on the availability of heat energy. However, the ease at which heat energy can be utilised is dependent on the temperature (or “level”) at which it is available. Devices such as heat pumps designed to “move” or “upgrade” heat from low temperature levels to higher temperature levels are well known to the applicant. However, they are of limited application. In addition, no heat pumps are available for pumping heat at elevated temperatures.
One of the inherent disadvantages of heat energy is that, because of its tendency to be transferred to bodies having a lower temperature, it cannot be stored easily for extended periods of time. This applies in particular to heat at a high temperature. Thus, the higher the temperature at which heat is to be stored, the greater its tendency to be lost due to transfer to lower temperature bodies.
This disadvantage can be substantially overcome or ameliorated by inhibiting the ease at which heat can be lost from a reservoir or body in which it is stored.
Heat transfer can take place by way of conduction, convection or radiation. It is well known that heat loss through conduction can be counteracted by the use of thermal insulation materials, that heat loss from a container through convection can be counteracted by applying a vacuum in a space provided between the container and an enclosure around the container (such as in a vacuum flask), and that heat loss through radiation can be counteracted by the use of materials having surfaces of which the emissivity is low.
Compared to heat energy, electrical energy is not suitable for storage at a large scale. Currently available technologies for storing electrical energy in the form of chemical energy, such as batteries, are very limited in size and have a limited life span. Other technologies such as flywheels, supercapacitors and fuel cells are also only being developed on a small scale. All are characterised by much lower energy densities than a heat storage system and a much higher cost.
A problem faced by large scale commercial producers of electricity is the need to install generating capacity substantially in excess of the average demand, because electricity generating plant must have sufficient capacity to meet peaks of demand. In Australia, for instance, there are typically two peaks per day, one occurring in the mornings and the other in the evenings. In normal times, the differences between peak and off-peak vary between about 125% of average demand and about 75% of average demand respectively. Thus, in NSW, Australia, for example, if the daily average consumption of electricity is around 7500 MW, the normal variation is from about 6000 MW off peak to about 12000 MW at peak. The magnitude of the peaks is usually related to weather conditions. Thus, very hot and very cold weather cause high consumption of electricity. In some other countries, particularly in the northern hemisphere, the variations between peak and off peak are much greater.
World-wide, the variations between peak and average in a particular country or region depend on a range of factors such as the extent to which households in the country or region are connected to electricity, the per capita electricity consumption in the country or region, the cost of electricity, the relative proportions of electricity consumed by industry, mining, agriculture and private households, etc. To discourage the use of electricity during peak periods and to encourage consumption during off-peak periods, utilities often charge a premium for electricity supplied during peak periods. Additionally or as an alternative, a maximum demand charge is sometimes levied, where such charge is related to the need for the utility to create sufficient generating capacity to cater for periods of peak demand.
Existing technologies for storing electrical energy on a large scale include technologies such as pumped storage hydro-electric schemes and compressed air systems. They are limited in the extent to which they can contribute to the smoothing out of supply between peak and off-peak periods, as they can only be installed where geographic features permit.
Since electrical energy cannot be easily stored as such, there is a need for a viable system for converting electrical energy to heat energy and for storing it in that form until it can be utilised at a later time.
There also exists a need for the storage of heat energy for relatively short periods of time between an off-peak period and the next peak period.
Although technologies exist for the recovery of energy from renewable sources, these technologies often suffer from the disadvantage that much of the recovered energy cannot be utilised when the energy is available whilst, when it is needed, the renewable source is not available. Thus, because of differences in the times when they are available and the times at which they are required, these renewable energies cannot be easily integrated into existing power grids. Technologies included in this category include those directed at the recovery of solar, wind and wave energy in the form of heat or electricity.
There accordingly exists a need for the better integration of renewable energies into power grids by relocating times of availability to times of demand as well as the ability to convert an intermittent energy supply into a permanent supply by storing the energy for use when the renewable energy is not available.
Because temperature is the driving force for heat transfer, there is also a need for a method and an apparatus for storing heat energy at high temperatures.
U.S. Pat. No. 4,089,176 describes a method and apparatus for operating power turbomachinery which includes a heat energy storage device comprising a graphite core. The graphite core is heated from its outside surface by electromagnetic induction. However, this storage device suffers from the disadvantage that energy losses are high, which is an inherent problem in heating a body of graphite from the outside, because of the higher operating temperatures on the outside surface of the body of graphite. The heat loss is high despite attempts to minimize it by the use of insulation. In addition, the temperature that can be tolerated on the outside surface of the core is limited by the maximum operating temperature of the insulation material.